Polyolefin resins are relatively inexpensive and have good properties such as chemicals resistance, water resistance, heat resistance, etc. Thus, they have been used as an industrial material in a wide range of applications; the use for auto parts is an example. Having those advantages, however, polyolefin resins are crystalline and have no reactive functional groups on their surface; so it has been difficult to apply adhesion and coatings to base materials of the polyolefin-based resin. To solve this problem, attempts have been made to improve the adhesiveness of coating films to polyolefin-based resins by modifying the resin surface through vapor cleaning, defatting, acid treatment, corona discharge treatment or plasma treatment. On the other hand, in the fields of the coatings, adhesives, and inks, chlorinated polyolefins, which have a relatively good adhesiveness to such hard-to-coat resin surfaces as mentioned above, have been used. However, most of those chlorinated polyolefins were of the solution type, dissolved in an organic solvent such as toluene, xylene, and/or esters. Thus, the organic solvents released into the atmosphere at the time of the coating operation gave a serious environmental and sanitary impact on coating workers and citizens living in a wide area around the coating place, and even on the surrounding ecosystem. Based on this situation, a patent application (WO9303104) has recently been made for an aqueous emulsion in which chlorinated polyolefins are blended with resins containing carboxylic acids. In these blend systems, however, a phase separation occurred between the different polymers after the formation of the coating film and the hydrophilic component, such as a surfactant came into the coating polymers, leading to the degraded strength and lowered water resistance of the coating film.
Considering what has been described above, the assignee of this patent has already completed the invention in which an aqueous emulsion is obtained by dissolving chlorinated polyolefins in monomers or oligomers and emulsifying the solution with a surfactant and then polymerizing (Japanese patent applications tokukaihei 5-209006, tokukaihei 5-287251, tokukaihei 6-287521, tokukaihei 8-176309). The object of these inventions was to increase the strength and water resistance of the coating film by copolymerization of chlorinated polyolefins with acrylic monomers to achieve the higher molecular weight. In order to achieve the polymerization as originally intended, however, the surfactant had to be selected from a narrow range and, in addition, it had to be used in a large amount. To improve the water resistance of the coating film from these chlorinated polyolefin-based resin emulsions, the use of water-soluble substances had to be further decreased.
The object of this invention is to achieve, without vapor-cleaning or defatting the surface of a polyolefin base material with chlorine-containing organic solvents such as trichloroethane, etc., a coating which has adhesiveness to a polyolefin base material and also has an improved interlayer adhesiveness to the base coat and/or the top coat and improved levels in properties such as water resistance, gasohol resistance, etc., and also which is less polluting or non-polluting.